Cryogenic liquid storage reservoirs usually comprise a double casing. The overflowing of the inner tank is a scenario with very serious consequences against which protection must be provided and which requires above all a very reliable detection. Specifically, the overflow of liquid in the annular insulation space of the cryogenic reservoir generates, by evaporation of the product placed in contact with hotter portions, a rise in pressure of the outer casing which may lead to damage to the reservoir and to a major accident.
In order to improve reliability, several types of detectors that are independent of one another are usually installed, but dependence on the individual reliability of the detectors remains. Each detector used must have an appropriate reliability, usually imposed by standards. For a detector operating continuously, the fact that it detects something and the accuracy of the detection may be sufficient to be convinced of the reliability. For a detector intended to detect an isolated phenomenon, such as the presence of a liquid in a given location or the exceeding of a high level, a test must be carried out.
The detectors of the prior art notably comprise slaved floating gauges which are tested by manually forcing them to rise, radars for which it is possible to simulate blocking out the beam, capacitors which cannot be tested in situ for the reasons cited above, or else floats which can be raised by hand.
Therefore, in liquid storage reservoirs, no level detection device of the prior art can be tested in real operating conditions without the physical level of liquid being intentionally raised to the detection threshold. On the understanding that the detection is designed to protect the reservoir against the accident described above, raising the level of liquid can be done only by filling. It therefore follows that, to test the alarm or protection device, the event initiating the feared phenomenon has to be created. If detection does not work, the consequences may be serious.
The object of the present invention is to remedy all or some of the drawbacks mentioned above, that is to say in particular to provide a liquid-presence detector that can be tested in situ in a realistic manner, without having to cause an event determining the detection and which, if it is not correctly detected, may have serious consequences.